First practice: amp died, help analysing problem?

[Slashpepper]

Just to keep everyone annoyed with my updates... I can't even get the back off... it's one of the MTS R212NXT's. How on earth do you remove the top 1/3rd? It's like it's glued in, let alone the rest of it. Can't get anything out to see what's going on inside.

Words cannot express my annoyance. Anyone help at all?

 

[Slashpepper]

Got it apart... Obviously never been taken apart before. That was effort, had to remove the side handles to apply enough force to unstick the back panel.

This wiring is really solidly put together, can't see anything wrong with any of it or how anything *could* go wrong. I'm actually impressed with the construction.

Still doesn't work though. How would one test the speakers themselves? They have clip on connectors but I have nothing I could attach to them.

Ideas anyone? Replacement speakers are going to be expensive aren't they.

 

[Julia]

check the impedance on each speaker with your multimeter. They should read the same.

Then someone else here will have some other clue.

You can order individual speakers from Avatar Speakers. Hellatones are just relabeled Celestions, and are around $89 - 99 ea. Make sure you get the correct ohm rating.

What were you doing to blow a speaker???

 

[kc2eeb]

Once you have checked the speaker impedance with a multimeter as Julia
suggests, (use the lowest ohm range possible; an 8 ohm speaker will read
6 ohms or so, a 16 ohm speaker 13-14 ohms or so.) If you get a good reading, remove the speaker from the cabinet. Place it magnet down on a table. Now, GENTLY, repeat, GENTLY, and NOT IN THE CENTER, push the speaker cone in and out with both hands. If you hear and/or feel a scraping
or scratchy sound, the voice coil is burnt. You can recone it or buy a new speaker.

 

[Slashpepper]

I'm getting my multimeter back from a friend tomorrow.

Using it normally! It doesn't make sense that it's blown it that badly, it sounds more like loose wires somewhere, but I can't find any.

It wasn't like I'd maxed the amp or anything... Slight possibility it might have been mismatched Ohms though, but I'm not sure. Amp at 16Ohm, Speakers at 8Ohm for about 2 hours. Even then, completely blowing them would surprise me.

Hopefully a wire's gone somewhere, that's easy!

 

[Slashpepper]

Urgh, right (silent) speaker measures 0.5 Ohm and the left hissy one measures 1.5 Ohm as measured on the two prongs to the top of the photo:

******.

Guess that means re-cone-ing or new speakers then? No wires that can come loose between there and the driver?

 

[kc2eeb]

Did you disconnect the speakers before you measured them?
They have to be disconnected from the output transformer to get the proper reading. Also, is the multimeter at it's lowest ohm setting?
And finally, check the meter by first touching the test leads together.
Depending on the meter, you should get the test lead resistence which is
about .05 to .2 ohms which you subtract from the reading you get from the speakers.

 

[kc2eeb]

Important!
The picture of the speaker has what looks like some solder touching
the frame of the speaker, right under the + (positive) terminal. If that solder is shorted (connected) to the positive speaker terminal, that's a problem! Check it with your meter.

 

[Slashpepper]

I'm glad you mentioned that because I meant to ask but forgot. I had not unplugged the speakers but have now... The leads have a resistance of between 0.1 and 0.4 Ohms, the right speaker is infinite and the left one 6.5 Ohms. Still not what I was hoping for though :-(

I think that solder's just a drip that's unconnected to anything, but will check properly when I'm back from work.

Is destroying two speakers completely something that's possible? It just seems... Unlikely... That I put that much power through them. I'm a little confused. *sigh*

 

[Slashpepper]

Oh and the meter was on the lowest setting, it only measures down to 0.1 Ohm accuracy so it was having trouble measuring the cable resistance...

 

[kc2eeb]

So, the left one is good and the right one is open so probably blown.
First thing to do is directly connect the left one to the amp and see how it sounds. It'll probably be OK. Then, take the right one out and trace the
wires from the terminals to the speaker cone. Sometimes, but rarely, there is a cold solder joint. Then, while you have the speaker out, gently push the cone as I previously described and see if it makes a scratchy
sound and/or you feel the voice coil rubbing. If the coil is blown, usually the insulation melts and thats what makes the scraping sound.

 

[Slashpepper]

Surely the fact that it's far below 16 Ohms is a bad sign? You can set the cab to stereo so that you can send a different input to each speaker. Even doing that has the weird distorted rustling guitar sound in the left speaker. Not sure how I'd connect it on its own without the electronics though, don't think I have any cables to do that...

 

[kc2eeb]

6.5 ohms is normal DC resistance for an 8 Ohm speaker.
Disconnect the leads from the right speaker and wrap some black
electricians tape over each separate end so they cannot touch each other.
I'm going onto the Randall site to see the manual for your cabinet.
O.K. I can't find any speaker manuals on the site.
If it has a stereo/mono switch does it have a separate left and right jack to connect the amp to?

 

[kc2eeb]

Can you post a picture of the input panel on the speaker cabinet?

 

[Slashpepper]

Unfortunately they're both 16 Ohm speakers :-(

It does indeed have seperate jacks, an input into either one has the same result as in both in mono mode. Mono gives a weird hiss out the left speaker, Stereo in the right speaker is silent and Stereo in the left gives the weird hiss.

There's definitely something up with the left speaker as well, although it could be a loose cable somewhere between the snap on clip in the first photo I posted and the actual driver, although I'm not sure I could get to it if that's the case without taking off the cone which would be the same cost as replacing it.

 

[Slashpepper]

Also when I've said left and right it's the opposite to how they're labelled, just to be confusing... Oops.

 

[kc2eeb]

OK. What I think is happening involves the mic eliminator circuit and the speaker impedances. While I have a RM50, a RT2/50, and a RM4, I don't have Randall cabinets. If I did I'd pull the panel and tell you what to check. My educated guess is that the speakers are 8 ohm and the eliminator circuit in series with the speakers give you 16 ohms per side in stereo. Celestion speakers, at least every one I've seen, have the impedance marked on them, usually on the rim. Are the speakers marked 16 ohms? If they are, then I'm wrong about the panel circuit.
The best test is to connect the speakers directly, either to the Randall head or to another amp.

 

[Slashpepper]

Hmm, possible I guess. I can't see the resistance labelled on the speakers anywhere unless it's hidden from view on the front. How annoying.

Unfortunately I don't have any cables that could connect them directly, I'd need some sort of clip to 1/4 inch jack. Hum. I'm wondering if I can test the resistance of the circuitry...

 

[Slashpepper]

Measuring between the back of the input jack and the end of the speaker wires give the same resistance as the test leads, so it would appear that they wouldn't add to the speakers (unless the electronics are broken, of course)

edit: Also, if it was a 8 Ohm circuit wouldn't it need massive heat sinks as it would take on half the power meant for the speakers?

 

[kc2eeb]

If it was an 8 ohm coil, like another speaker coil, it would dissipate the power in a way similar to the speakers. Some speaker emulators were, and probably still are, a speaker with the cone removed and the voice coil epoxied in place in the magnet structure, so you get a response electronically like a speaker without the sound. The coil can't move so it's not identical and (without getting into a lot of theory) it does not produce the same ac voltage feedback to the amplifier, which all moving coil speakers do.
The easiest thing to do is to test the speaker directly connected to the amp.
I don't know how good you are with soldering, but all you would need is a piece of ac "zip cord," nothing special, the same wire a lamp uses, two
alligator clips and a phone plug, all Home Depot and Radio Shack. Or, can someone solder it for you?